Sky Quality Meter. Any good?

[Petrol]

I live in the UK in a very wet and damp area. Living in a valley doesn't help either because it's often misty. Sometimes I travel in my small RV to higher, darker skies but how good are they? I don't really understand the implications of sky quality magnitudes and I apprieciate it varies from night to night. Magnitudes per square arcseconds mean nothing to me. That said, if I had a hand held device to point at the sky to give me a rough idea of how good the sites I visit are, it might save me driving long distances for nothing.
I've been looking at the Unihedron SQM-L which at about 125 bucks looks good, then there's the Pegasus Uranus 2 Meteo Sensor at about 350, bit too much really. If anyone can offer me some basic advice I'd really appriciate it.

Pete

[Bigzmey]

I believe Alan uses one and can comment @kt4hx

But you don't really need it to tell how good is the sky. If the Milky Way is easily visible then the sky is good for most observing and AP purposes. If the Milky Way spans horizon to horizon, bright and well structured, then you got really good sky.

[kt4hx]

As Andrey mentioned, I have the SQM-L. MPSAS is how the brightness of the sky is measured. In terms of sky darkness, a reading of 22.0 MPSAS with the meter pointed at the zenith would indicate the darkest skies you would encounter. While a reading of 16.0 would indicate the brightest you could encounter. MPSAS is basically a scale of sky luminance.

In my case, at our dark site property I have encountered as dark as around 21.6, but on average nights it tends to be around 21.3, which is a nice sky to observe under. You are correct that it can vary over the course of an evening as conditions in the sky change. Aiming at the summer Milky Way band will give you a lower (brighter) reading, so one has to be cognizant what is in the direction of where it is aimed. While it is a pretty good tool for getting a sense of the sky's quality at a given location at a given time, it is not something that I use on a regular basis. I can get a pretty good sense of what the sky is like easily while I am observing. Then again, I do not travel to various locations. I either observe from home, which is typically around 19.2 MPSAS on the meter or at the other property, which averages about 21.3 as I mentioned.

I don't know if you have heard of the Bortle Scale, devised by John Bortle back around 2000, and first published in Sky & Telescope magazine. It is a method for estimating sky quality in real time, utilizing your eyes only, based on a set of criteria that John established. It can be found here: https://skyandtelescope.org/astronomy-r ... sky-scale/

One's Bortle quality can vary over the course of an evening as conditions shift. While it is a useful tool, it is not foolproof or easily translated from one person to the next because of individual experience levels and personal optical acuity.

Another method utilized by observers is the colored light pollution maps. These are created by using nighttime satellite imagery with an algorithm applied to estimate the spread of light glow from populated areas across terrain. They utilize a colored scale to indicate the level of "estimated" light pollution in particular locations. These can be a broadly useful tool to locate areas that are possibly darker. However, localized conditions can vary, and there can be light sources that might impact a specific geographic location that might not be accounted for in the broader scale of the maps. Also, locations nearer water, be that lakes, streams or ponds, might be more prone to fog, which is not factored into the colored maps. Even after identifying potential sites, they still need to be vetted to make certain they live up to the assumed quality. Here is one example of the mapping technology: https://www.lightpollutionmap.info/#zoo ... kiOjg1fQ==

There are various sites that attempt to correlate the Bortle Scale to the colored maps, but that is not with the blessing of John Bortle. I can tell you he does not care for such attempts to combine his scale with the LP mapping trend. So I would take such attempts with a grain of salt. Truth be told the two types utilize differing data and methodologies, so they don't necessarily play well together.

Anyway, I hope some or all of this helps. Quite frankly, whether one has an SQM, uses the Bortle Scale or the colored LP maps, the ultimate indicator of the sky quality for where you are at the time you are there, are you eyes and common sense.

[Juno16]

Hi Pete,

Alan’s post above is excellent!

I live in an area significantly impacted by poor sky conditions and light pollution.

I posted this earlier in the year.

viewtopic.php?p=238879#p238879

Since you are an imager, you probably have existing data you can analyze. You might already use ASTAP for plate solving.
This method worked well for me to establish a baseline.
Be aware that ASTAP recommends that images to be analyzed should be captured using a uv/ir cut filter for best results.
Hopefully this will be helpful.

[kt4hx]

Hi Pete,

Alan’s post above is excellent!

I live in an area significantly impacted by poor sky conditions and light pollution.

I posted this earlier in the year.

viewtopic.php?p=238879#p238879

Since you are an imager, you probably have existing data you can analyze. You might already use ASTAP for plate solving.
This method worked well for me to establish a baseline.
Be aware that ASTAP recommends that images to be analyzed should be captured using a uv/ir cut filter for best results.
Hopefully this will be helpful.

Good points Jim. Since I am purely a visual observer, sky quality has a more direct impact for me. What my eye sees, it sees. With imaging, one can mitigate certain aspects associated with LP and pull out more than what the eye can see under the same conditions.

[JayTee]

I have one. I use it and I believe it. When it's current reading is translated and compared to a comperable NELM number, they almost always agree.

[Petrol]

Thanks everyone for your replies and information, it's really appreciated. The skies where I live have detiriorated in the last 10 years. I used to look up and could see the double cluster as a fuzzy patch regularly. I haven't seen it for a few years and the sky glow has definitely increased since the introduction of LED street lighting or maybe more general illumination. I've seen it when I travel in my RV to image though. 30 minutes to an hour and I go from bortle 5 to bortle 4 according to the light pollution maps. If I travel 3 hours I can get to bortle 3.
I fully appreciate that my eyes can see more than a budget device and it will only give a reading of sky glow. Humidity or high cloud can effect what you can see just as much as sky glow but for the cost of it, I've bought one. The plan is to visit various locations and take readings over several nights / conditions. This, combined with what I can see, should give me a good idea of how far I need to travel to get some of the more difficult targets. Guess I was spoiled when I was in Texas under Bortle 1 skies with virtually zero high cloud and crystal clear skies.
Again thanks,
Pete

[AstroBee]

I too have a Unihedron SQM-L and use it every time I travel to dark sites. It's nice to know what the current sky conditions are. I usually take a reading several times throughout the night and then average them to get a # for that particular location.
The nice thing about the Pegasus Uranus 2 Meteo Sensor is that if you use other Pegasus gear, like I do with the Pegasus Astro Ultimate Powerbox v2 and the Unity software, you can have it automatically capture readings at set times and it will give you a more accurate idea of how your sky conditions are changing throughout the evening. It adds the data to the log that it outputs on shutdown.